The invention relates to an electric lamp comprising:
a light source provided with a light-transmitting chamber,
a first and a second conductive wire connected to a first and a second end of the light source, respectively, thus forming a conduction path with the light source,
an outer sealed envelope formed from a light-transmitting material and surrounding said chamber and said conductive wires, the environment between the outer envelope and said chamber being substantially without oxygen.
Such a lamp is described in the UK patent application GB 2,272,569. It provides a tungsten lamp which has a light-transmitting filament chamber enclosing a filament, said filament chamber being enclosed within an outer envelope. The lamp described in the prior art comprises a light source consisting in the filament chamber enclosing the tungsten filament. During lamp operation, the temperature of the light source increases strongly and could ignite any item in its neighborhood. In the event of a fracture of the outer envelope of the lamp proposed by the prior art, the light source does not necessarily stop functioning. Thus, there is a high probability that the light source at a very high temperature will ignite an external item touching it.
It is an object of the invention to provide a lamp whose light source will stop functioning as soon as the outer envelope of the lamp breaks, thus offering a high degree of safety to any external user.
To this end, the lamp described in the introduction is provided with at least a portion of at least a conductive wire, hereinafter referred to as a safety cut-out wire, made of a material which reacts chemically when in contact with air, causing the conduction path to open.
In normal operation, a conduction path is established along the first conductive wire, the light source, and the second conductive wire. The space between the outer envelope and the light source chamber is very poor in oxygen and may be vacuum or filled with a gas which does not react with any component inside the lamp. The portion of the safety cut-out wire is made of a material which reacts as soon as it is in contact with air and is chemically transformed in such a way that the conduction path is rapidly broken and the light source no longer functions. An advantage of a lamp according to the invention is that it offers a good protection against a possible ignition of an external item through contact with the light source when the outer envelope is broken or cracked. Fuses are described in the lamp of the prior art. They are connected to opposite ends of the filament to limit the magnitude and duration of an electrical arc across a break in the filament. These fuse wires are designed to melt and break the electrical contact in the event of an arc. The safety cut-out wire introduced by the invention deals with a different problem than do these fuse wires. The safety cut-out wire is not a protection against an electrical arc which may arise across a break in the filament and does not melt at high temperatures.
In a preferred embodiment of the invention, the material of said portion of the safety cut-out wire burns out when in contact with air at a temperature reached owing to radiation heat from the light source during operation.
Indeed, a light source in operation reaches very high temperatures and, as a consequence, so may the conductive wires when placed in the vicinity of the light source. Many commonly used materials burns out in a few seconds in oxygen at such a temperature level. Thus the protection provided by the safety cut-out wire can be implemented in a simple way by using common and inexpensive materials. Besides, in another embodiment of the invention, the safety cut-out wire may also be heated by the Joule effect due to a current circulating in the conductive wires.